Getting a charge out of backpacking.

Talk about plugging along. Soldiers, emergency workers, or hikers toting heavy loads may soon don a new kind of backpack that converts energy from their strides into electricity. The souped-up backpack, which produces power from the up-and-down oscillations of the wearer's body, can supply juice to electronic gadgets a hiker might have along.

That could be a boon for participants in numerous outdoor endeavors, says inventor Lawrence C. Rome of the University of Pennsylvania in Philadelphia and the Marine Biological Laboratory in Woods Hole, Mass. In military actions, search-and-rescue operations, and scientific field studies, people rely increasingly on cell phones, global positioning system receivers, night-vision goggles, and other battery-powered devices. The backpack's electricity-generating feature could dramatically reduce the fraction of a wearer's load now devoted to spare batteries, report Rome and his colleagues in the Sept. 9 Science.

To derive electricity from human locomotion, the developers used springs to suspend a cloth pack from an external frame. A gear mechanism converts vertical movements of the pack to rotary motions of an electrical generator, producing up to 7.4 watts.

Unexpectedly, tests showed that wearers of the new backpack alter their gaits in response to the pack's oscillations, so that they carry loads more comfortably and with less effort than they do ordinary backpacks. Because of that surprising advantage, Rome plans to commercialize both electric and non-electric versions of the backpack.--P.W.